I observed this swimming Cottonmouth early one predawn just as the sky was starting to lighten. The visibility at this time of morning was not very good due to the relative darkness, and that is reflected in the quality of these photographs. Still, there is enough here to positively identify this snake as a Cottonmouth and not some other water snake.
Cottonmouths typically hold their heads at a 45 degree angle whether they are swimming or just crawling across the ground. That is clearly the case in these pictures. Further this snake is also very buoyant, and is swimming with almost its entire body on the surface of the water—this is also very typical of Cottonmouths. Other nonvenomous water snakes swim under water or with only their heads exposed.